The gonadotropins are a group of heterodimeric glycoprotein hormones which regulate gonadal function in the male and female. They include follicle stimulating hormone (FSH), luteinising hormone (LH) and chorionic gonadotropin (CG).
Human chorionic gonadotropin (hCG) is naturally secreted by the anterior pituitary gland and functions to support follicular development and ovulation. hCG comprises a 92 amino acid alpha sub-unit, also common to the other glycoprotein hormones LH and FSH, and a 145 amino acid beta sub-unit unique to hCG, which dictates the hormone specificity. Each sub-unit is post translationally modified by the addition of complex carbohydrate residues. The alpha sub-unit contains 2-N-linked glycosylation sites at amino acids 52 and 78 and the beta sub-unit contains 2-N-linked glycosylation sites at amino acids 13 and 30 and four O-linked glycosylation sites at amino acids 121, 127, 132 and 138.
hCG extracted from the urine of pregnant women [CHORAGON® (Ferring)] has been used for many years in infertility treatment. The production of hCG extracted from urine involves the collection and processing of large amounts of urine. A recombinant version of hCG, OVITRELLE® (Serono), is available. This is expressed in Chinese hamster ovary (CHO) cells. The known recombinant hCG product has a different pharmacokinetic profile to hCG produced from humane urine. It is desirable to have an hCG product that more closely replicates or mimics the pharmacokinetic profile of the product produced from human urine.
There is considerable heterogeneity associated with hCG preparations which relates to differences in the amounts of various isoforms present. Individual hCG isoforms exhibit identical amino acid sequences but differ in the extent to which they are post-translationally modified; particular isoforms are characterised by heterogeneity of the carbohydrate branch structures and differing amounts of sialic acid (a terminal sugar) incorporation, both of which appear to influence the specific isoform bioactivity.
Glycosylation of natural hCG is highly complex. The glycans in naturally derived pituitary hCG can contain a wide range of structures that can include combinations of bi-, tri- and tetra-antennary glycans. The glycans can carry further modifications: core fucosylation, bisecting glucosamine, chains extended with acetyl lactosamine, partial or complete sialylation, sialylation with α2,3 and α2,6 linkages, and sulphated galactosamine substituted for galactose. Furthermore, there are differences between the distributions of glycan structures at the individual glycosylation sites.
The glycosylation of recombinant hCG (“rhCG”) products reflects the range of glycosyl-transferases present in the host cell line. The existing rhCG product, OVITRELLE®, is derived from engineered Chinese hamster ovary cells (CHO cells). The range of glycan modifications in CHO derived rhCG are more limited than those found on the natural products, derived from urine. Examples of the reduced glycan heterogeneity found in CHO derived rhCG include a lack of bisecting glucosamine and a reduced content of core fucosylation and acetyl lactosamine extensions. In addition, CHO cells are only able to add sialic acid using the α2,3 linkage (Kagawa et al., 1988, Takeuchi et al., 1988, Svensson et al., 1990). This is different from naturally produced hCG which contains glycans with a mixture of α2,3 and α2,6-linked sialic acid.
It has been demonstrated that a recombinant FSH preparation (Organon) differs in the amounts of FSH with an isoelectric point (pI) of below 4 (considered the acidic isoforms) when compared to pituitary, serum or post-menopausal urine FSH (Ulloa-Aguirre et al., 1995). The amount of acidic isoforms in the urinary preparations of FSH was much higher as compared to the recombinant products, GONAL-F® (Serono) and PUREGON® (Organon) (Andersen et al., 2004). This must reflect a lower molar content of sialic acid in rFSH since the content of negatively-charged glycan modified with sulphate is low in FSH. The lower sialic acid content, compared to natural FSH, is a feature of both commercially available FSH products and therefore must reflect a limitation in the manufacturing process (Bassett and Driebergen, 2005). The circulatory life-time of FSH has been documented for materials from a variety of sources. Some of these materials have been fractionated on the basis of overall molecular charge, as characterised by their pI, in which more acid equates to a higher negative charge. The major contributor to overall molecular charge is the total sialic content of each FSH molecule. For instance, rFSH (Organon) has a sialic acid content of around 8 mol/mol, whereas urine-derived FSH has a higher sialic acid content (de Leeuw et al. 1996). The corresponding plasma clearance rates in the rat are 0.34 and 0.14 ml/min (Ulloa-Aguirre et al., 2003). In another example where a sample of recombinant FSH was split into high and low pI fractions, the in vivo potency of the high pI (lower sialic acid content) fraction was decreased and it had a shorter plasma half-life (D'Antonio et al., 1999). The applicants have found that, similar to FSH, the known, CHO derived, recombinant hCG product (e.g., OVITRELLE®) also has a lower amount of hCG with an isoelectric point (pI) of below 4 (considered the acidic isoforms) than urinary hCG, also reflecting a lower sialic acid content of the known rhCG product compared to urinary hCG.
The total sialic acid content of hCG and rhCG is not directly comparable since sialic acids are commonly linked in two ways. Pituitary/serum/urinary hCG contain both α2,3 and α2,6-linked sialic acid, with a predominance of the former. However, CHO cell derived recombinants only contain α2,3 (Kagawa et al., 1988, Takeuchi et al., 1988, Svensson et al., 1990). In other words, recombinant proteins expressed using the CHO system will differ from their natural counterparts in their type of terminal sialic acid linkages. This is another difference between natural and current recombinant products in addition to the lower overall sialic acid content of the latter, and is an important consideration in the production of biologicals for pharmaceutical use since the carbohydrate moieties may contribute to the pharmacological attributes of the molecule.